linux-kernel - Re: Ordering between PCI config space writes and MMIO reads?

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Message-ID: <45494515.8050304@sgi.com>
Date:	Wed, 01 Nov 2006 19:08:37 -0600
From:	John Partridge <johnip@....com>
To:	David Miller <davem@...emloft.net>
CC:	rdreier@...co.com, matthew@....cx, jmodem@...minableFirebug.com,
	mst@...lanox.co.il, linux-kernel@...r.kernel.org,
	linux-ia64@...r.kernel.org, jeff@...zik.org,
	openib-general@...nib.org, linux-pci@...ey.karlin.mff.cuni.cz
Subject: Re: Ordering between PCI config space writes and MMIO reads?

David Miller wrote:
> From: John Partridge <johnip@....com>
> Date: Wed, 01 Nov 2006 10:27:19 -0600
> 
> 
>>Sorry, but I find this change a bit puzzling. The problem is
>>particular to the PPB on the HCA and not Altix. I can't see anywhere
>>that a PCI Config Write is required to block until completion, it is
>>the driver and the HCA ,not the Altix hardware that requires the
>>Config Write to have completed before we leave mthca_reset()
>>Changing pci_write_config_xxx() will change the behavior for ALL
>>drivers and the possibility of breaking something else. The fix was
>>very low risk in mthca_reset(), changing the PCI code to fix this is
>>much more onerous.
> 
> 
> The issue is that something as simple as:
> 
> 	val = pci_read_config(REG);
> 	val |= bit;
> 	pci_write_config(REG, val);
> 	newval = pci_read_config(REG);
> 	BUG_ON(!(newval & bit));
> 
> is not guarenteed by PCI (aparently).
> 
> I see no valid reason why every PCI device driver should
> be troubled with this lunacy and the ordering should thus
> be ensured by the PCI layer.
> 
> It just so happens to take care of the original driver
> issue too :-)

Yeah, Matthew has convinced me of that now.

Thanks


-- 
John Partridge

Silicon Graphics Inc
Tel:	651-683-3428
Vnet:	233-3428
E-Mail:	johnip@....com
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